Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field (B0 field) whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system in which the measurement is based. The magnetic field splits different energy levels for the individual nuclear spins in dependence on the magnetic field strength and the specific spin properties. The spin system can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as B1 field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an above mentioned electromagnetic pulse of appropriate radio frequency (RF pulse) while the corresponding B1 magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic RF pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°).
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of one or more receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The MR signal data obtained via the RF coils corresponds to the spatial frequency domain and is called k-space data. The k-space data are usually acquired along multiple lines with different phase encoding values to achieve sufficient coverage. Each line is digitized during read-out by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation.
With aging population and an increasing number of patients carrying metal implants, the need for MR imaging of soft tissue in the presence of metal increases. Metal resistant MR imaging is required to enable imaging this soft tissue for diagnosis of complications and follow-up after surgery. MR imaging near metal is typically compromised by susceptibility issues degrading the magnetic fields used for image formation locally. In diagnostic MR imaging scans, the susceptibility of the metal parts causes MR signal pile-up, signal voids and other geometric distortions. Multispectral imaging techniques like SEMAC (Lu et al, ISMRM 2008, p. 838) and MAVRIC (Koch et al, ISMRM 2008, p. 1250) have been proposed to counter susceptibility issues in diagnostic MR imaging scans at the cost of increased scan duration, which scales with the required frequency coverage.
Known parallel acquisition techniques can be used for accelerating the multispectral MR signal acquisition. A method in this category is SENSE (Sensitivity Encoding). SENSE and other parallel acquisition techniques use undersampled k-space data acquisition obtained from multiple RF receiving coils in parallel. In these methods, the (complex) signal data from the multiple RF receive coils are combined with complex weightings in such a way as to suppress undersampling artifacts (aliasing) in the finally reconstructed MR images. This type of complex RF coil array signal combination is sometimes referred to as spatial filtering and includes combining in the k-space domain or in the image domain (in SENSE), as well as methods which are hybrids. In SENSE imaging, coil sensitivity profiles are typically estimated from low-resolution reference data obtained by a SENSE reference scan. This coil sensitivity information is then used to “unwrap” aliased pixels in image space using a direct inversion algorithm.
The present standard for the SENSE reference scan is a gradient echo sequence, namely a FFE (Fast Field Echo=gradient echo with small flip-angle excitation) acquisition protocol, which makes it very sensitive to susceptibility effects. When the susceptibility of metal parts compromises the quality of the SENSE reference scan, it may cause SENSE unfolding problems and signal voids resulting in an insufficient quality of the finally reconstructed MR images.
Conventionally, the MR device employed for a given diagnostic imaging task automatically detects when a SENSE reference scan is required depending on the type and the parameters of the selected imaging sequence. The SENSE reference scan is automatically inserted into the list of sequences to be performed, typically immediately before the diagnostic imaging sequence.
Using a spin echo sequence, namely a TSE (Turbo Spin Echo=spin echo with multiple 180° refocusing RF pulses) SENSE reference scan has been shown to be more robust against susceptibility effects, thereby reducing problems like incorrect SENSE unfolding and signal voids. However, a TSE SENSE reference scan may take a full minute or more, which is substantially longer than the standard FFE SENSE reference scan, which takes typically less than 10 seconds.
It is generally not known a priori if a patient to be examined has a metal implant or not. It would be inefficient to precautionary employ a metal resistant SENSE reference scan in order to avoid image artefacts. In many cases, a conventional SENSE reference scan is sufficient.
From the foregoing it is readily appreciated that there is a need for an efficient MR imaging technique that is robust against susceptibility effects.